1. Field of the Invention
The present invention relates to a blood pressure monitor system which continuously measures intra-arterial blood pressure of a living subject and particularly relates to the art of improving the accuracy of blood pressure measurement.
2. Related Art Statement
There has been proposed a blood pressure monitor system including a pressure sensor having a press surface and including one or more pressure sensing elements provided in the press surface; a pressing device which presses the pressure sensor against an arterial vessel of a living subject such as a patient via a body surface of the subject so that each pressure sensing element of the pressure sensor measures pressures at the body surface of the subject; pressing-force determining means for determining an optimum pressing force of the pressing device at which a portion of a wall of the artery is flattened under the pressure sensor pressed by the pressing device; and blood-pressure determining means for operating the pressing device to maintain the determined optimum pressing force and press the pressure sensor against the artery via the body surface or skin, and continuously determining intra-arterial blood pressure values of the artery, based on the pressure magnitudes or values measured by the pressure sensor at the body surface. An example of this monitor system is disclosed in U.S. Pat. No. 5,119,822 or U.S. Pat. No. 5,179,956.
In the above-indicated prior monitor system, the pressure sensor is pressed against the artery via the body surface or skin, such that the wall of the artery is partly flattened under the pressure sensor. Since the pressure values measured by the pressure sensor through the flattened wall of the artery are free from adverse influences of the tensile forces produced in the arterial wall, they well reflect intra-arterial blood pressure values of the artery. According to this blood pressure measurement principle, the prior monitor system continuously measures the blood pressure of the subject by using the pressure sensor pressed at the optimum pressing force.
Meanwhile, the experiments the present inventors conducted have elucidated that the blood pressure values continuously measured by the above-indicated prior monitor system tend to be higher than the blood pressure values measured using an inflatable cuff, and do not enjoy sufficiently high measurement accuracy. In this background, the present inventors have made various studies and experiments, and found that the soft and elastic subcutaneous tissue exists between the arterial vessel and the pressure sensor and that a "provisional" blood pressure measured by the pressure sensor, i.e., pressure sensing element positioned directly above the artery and pressed at the optimum pressing force contains both a "true" intra-arterial blood pressure of the artery and an "additional" pressure added thereto because of the elastic force of the subcutaneous tissue under the pressure sensor.